Disclaimers: (1) This key isn't literally dichotomous. (2) It is intended for use with the 41 freshwater gastropod species confirmed for the Atlantic drainages of Delaware, Maryland, New Jersey, Pennsylvania, and the West Virginia panhandle. The management will not be responsible for any loss, damage, or injury resulting from its application elsewhere.

1a) Operculum present. . . Subclass Prosobranchia (2)
1b) Operculum absent . . . Subclass Pulmonata (17)

2a) Operculum calcareous . . . Bithynia tentaculata
2b) Operculum corneous . . . (3)

3a) Operculum multispiral [photo] . . . (4)
3b) Operculum concentric [photo] . . . (5)
3c) Operculum paucispiral [photo] . . . (9)

4a) Shells typically almost planispiral, bearing two carinae . . . Valvata bicarinata
4b) Shells bearing 3 carinae . . . Valvata tricarinata 

5a) Shell with color bands . . . Viviparus georgianus
5b) Shell unbanded . . . (6)

6a) Adults strikingly large, shells greater than 40 mm standard length . . . (7)
6b) Adults smaller  . . . (8)

7a) Shell uncarinate, aperture lip typically purple . . . Cipangopaludina chinensis
7b) Shell carinate, especially noticeable in juveniles, aperture never
     purple . . . Cipangopaludina japonica  [photo]

8a) Shell smooth . . . Campeloma decisum
8b) Shell bearing fine spiral cords . . . Lioplax subcarinata

9a) Adults over 10 mm shell length . . . Family Pleuroceridae (10)
9b) Adults under 10 mm shell length . . . (12)

10a) Shell height less than 1.7 times width . . . Leptoxis carinata
10b) Shell height greater than 1.7 times width . . . (11)

11a) Shell bearing many fine spiral cords, at least on
      early whorls . . . Pleurocera virginica
11b) Shell with just a single carination,
      typically becoming obsolete on later whorls . . . Pleurocera proxima

12a) Primarily terrestrial in life habit, found above the
      water level . . . Pomatiopsis lapidaria
12b) Entirely aquatic in life habit . . . Family Hydrobiidae (13)

13a) Penis with a single duct . . . (14)
13b) Penis with two ducts . . . (16)
13c) Males rare or entirely absent, females parthenogenic . . . Potamopyrgus antipodarum

14a) Penis simple, unlobed . . . Somatogyrus pennsylvanicus
14b) Penis fringed with numerous papillae . . . Littorinidops tenuipes
14c) Penis with two accessory lobes . . . (15)
14d) Penis with three accessory lobes . . . Fontigens bottimeri

15a) Proximal penial lobe containing tubular gland . . . Fontigens nickliniana
15b) Proximal penial lobe containing bulbous gland . . . Fontigens orolibas

16a) Operculum entirely paucispiral . . . . Amnicola limosa
16b) Operculum initially multispiral, becoming paucispiral . . . Lyogyrus granum

17a) Shell dextral . . . Family Lymnaeidae (18)
17b) Shell sinistral, not planispiral . . . Family Physidae (21)
17c) Shell sinistral, planispiral . . . Family Planorbidae (25)
17d) Shell patelliform . . . Family Ancylidae (31)

18a) Adult shell length 10 mm or less . . . Lymnaea humilis
18b) Adult shell length greater than 10 mm . . . (19)

19a) Aperture less than 60% shell height . . . Lymnaea elodes
19b) Aperture greater than 60% shell height . . . (20)

20a) Populations on rocks & solid substrates in lakes and rivers . . . Lymnaea catascopium
20b) Populations amphibious, on emergent vegetation . . . Lymnaea columella

21a) Penis lacks a preputial gland . . . Aplexa elongata
21b) Penis bearing a preputial gland . . . (22)

22a) One-part, muscular penial sheath . . . Physa acuta
22b) One-part, glandular penial sheath . . . Physa vernalis
22c) Two-part penial sheath . . . (23)

23a) Shell slender, body generally dark . . . Physa carolinae
23b) Animal lightly pigmented, shell with shoulders more pronounced . . . (24)

24a) Adults greater than 7 mm shell length, whorls convex . . . Physa gyrina
24b) Adults less than 7 mm shell lenght, whorls not notably convex . . . Physa pomilia

25a) Adult greater than 10 mm shell diameter . . . (26)
25b) Adult less than 10 mm shell diameter . . . (28)

26a) Adult lip flared . . . Helisoma campanulatum
26b) Adult lip not flared . . . (27)

27a) Spire appears deeply indented when viewed from either aspect . . . Helisoma anceps
27b) Spire appears flattened when viewed from one aspect . . . Helisoma trivolvis

28a) Shell covered with periostracal hairs . . . Gyraulus deflectus
28b) Shell bare, with rounded periphery . . . (29)
28c) Shell bare, periphery carinate . . . (30)

29a) Adults smaller than 4 mm maximum shell dimension . . . Gyraulus parvus
29b) Adults larger than 4 mm maximum shell dimension . . .Planorbula armigera

30a) Carina strong, approximately mid-whorl . . . Promenetus exacuous
30b) Carina weak, distinctly off mid-whorl . . . Menetus dilatatus

31a) Apex entirely smooth . . . Laevapex fuscus 
31b) Apex bearing fine radial striae . . . (32)

32a) Apex distinctly to the right of the midline [photo] . . . Ferrissia fragilis
32b) Apex approximately in the midline [photo] . . . Ferrissia rivularis